System and method for stabilizing unintentional muscle movements

ABSTRACT

A system and method for stabilizing a position of an object are disclosed. The system comprises a housing that includes a subsystem. The system also includes an attachment arm coupled to the housing. At least one first sensor is placed along the attachment arm, wherein the attachment arm is configured to receive the object thereto. In response to an unintentional muscle movement by a user that adversely affects the motion of the object, the subsystem stabilizes the position of the object. The method comprises providing a subsystem within a housing and coupling an attachment arm to the housing. The method also includes placing at least one first sensor along the attachment arm, wherein the attachment arm is configured to receive the object thereto. In response to an unintentional muscle movement by a user that adversely affects the motion of the object, the subsystem stabilizes the position of the object.

This invention was made with government support under Grant No. NS070438awarded by National Institutes of Health (NIH). The government hascertain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to unintentional musclemovements of a body, and more particularly, to a system and method forstabilizing the effects of these unintentional muscle movements.

BACKGROUND

Unintentional muscle movements of the human body, or human tremors, canoccur in individuals suffering from neurological motion disordersincluding but not limited to Parkinson's Disease (PD) and EssentialTremor (ET). ET is the most common neurological motion disorderaffecting as many as 10 million individuals in the United States and 270million individuals worldwide. Due to the debilitating muscle movementsassociated with this disease, individuals with ET have difficulty inperforming many daily functions such as eating and drinking. As aresult, these individuals often suffer from social isolation,depression/anxiety, and an overall reduced Health Related Quality ofLife (HRQoL).

Unintentional muscle movements of the human body can also occur inhealthy individuals. These unintentional muscle movements are oftenexacerbated by environmental factors and situations that lead tofatigue, stress, nervousness, etc. For example, a military servicepersonmay experience unintentional muscle movements while performing asurgical operation on the battlefield due to stress and nervousness andthis may result in decreased performance.

For individuals suffering from neurological motion disorders, a varietyof treatment options exist. Pharmacological treatments vary ineffectiveness, can lead to severe side effects and are unable to slow orstop disease progression. Surgical procedures, such as Thalamotomy andthalamic Deep Brain Stimulation (DBS) can be expensive, dangerous, andlimited in availability. Non-invasive solutions, such as physicallygrounded tremor suppression devices, physically force a person's tremorto cease but require complex and costly structures, cause userdiscomfort and cannot differentiate between intended and unintendedmovements.

These issues limit the adoption of these treatments to selectneurological motion disorder cases. Also, these treatments are often notavailable for healthy individuals suffering from human tremor. Thus, forthe majority of individuals that suffer from human tremor, there is astrong need for a non-invasive solution that overcomes the above issues.The present invention addresses such a need.

SUMMARY OF THE INVENTION

A system and method for stabilizing a position of an object aredisclosed. In a first aspect, the system comprises a housing. Thehousing includes a subsystem. The system also includes an attachment armcoupled to the housing. At least one first sensor is placed along theattachment arm, wherein the attachment arm is configured to receive theobject thereto. In response to an unintentional muscle movement by auser that adversely affects the motion of the object, the subsystemstabilizes the position of the object.

In a second aspect, the method comprises providing a subsystem within ahousing and coupling an attachment arm to the housing. The method alsoincludes placing at least one first sensor along the attachment arm,wherein the attachment arm is configured to receive the object thereto.In response to an unintentional muscle movement by a user that adverselyaffects the motion of the object, the subsystem stabilizes the positionof the object.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate several embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. One of ordinary skill in the art readilyrecognizes that the particular embodiments illustrated in the drawingsare merely exemplary, and are not intended to limit the scope of thepresent invention.

FIG. 1 illustrates a conventional handheld system that detects andcompensates for unintentional muscle movements.

FIG. 2 illustrates a system that detects and compensates forunintentional muscle movements in accordance with an embodiment.

FIG. 3 illustrates a motion-generating mechanism in accordance with anembodiment.

FIG. 4 illustrates an analytical model in accordance with an embodiment.

FIG. 5 illustrates a system diagram of the control system in accordancewith an embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates generally to unintentional musclemovements in a body, and more particularly, to a system and method forstabilizing the effects of these unintentional muscle movements. Thefollowing description is presented to enable one of ordinary skill inthe art to make and use the invention and is provided in the context ofa patent application and its requirements. Various modifications to thepreferred embodiment and the generic principles and features describedherein will be readily apparent to those skilled in the art. Thus, thepresent invention is not intended to be limited to the embodiments shownbut is to be accorded the widest scope consistent with the principlesand features described herein.

FIG. 1 illustrates a conventional handheld system 100 that detects andcompensates for unintentional muscle movements. The handheld system 100includes a base 102, a gripping element 106 coupled to the base 102, andan object 116 (in this embodiment, a spoon) coupled to the grippingelement 106. The base 102 houses a stabilizing assembly using shapememory alloy (SMA) wires 104, a power source 108 coupled to thestabilizing assembly 104, a single sensor 110 coupled to the powersource 108, a controller 112 coupled to the single sensor 110, and ashaft 114 coupled to the stabilizing assembly 104. SMA wires are alloywires that, after deformation, undergo a phase change to return to theiroriginal cold-forged shape after sufficient heat is applied. The SMAwires utilized in the stabilizing assembly 104 are heated by the powersource 108 to trigger this phase change.

In the handheld system 100, the single sensor 110 is located within thebase 102 to detect a user's motion and then the sensor 110 commands thestabilizing assembly using SMA wires 104 to produce a canceling motion.Unfortunately, several problems exist preventing the immediate use ofSMA wires. For example, SMA wires have not been proven for long-term,reliable use and also require significant complexity and cost to providesufficient motion to cancel large amplitude (1-4 cm) disabling tremors.

In addition, because the single sensor 110 is located within the base102, the use of the device is restricted to an object 116 that has apre-determined length and weight that must be pre-programmed into thecontroller 112. Deviations from this pre-determined length or weightwill result in control instabilities and a reduction in the efficacy ofthe motion cancellation.

A system and method in accordance with the present invention addressesthese drawbacks. The system and method include an inertial sensor placedalong an attachment arm and a motion-generating mechanism that does notutilize SMA wires. In so doing, the motion of the varying stabilizedobjects can be directly measured and there is no need forpre-programming the pre-determined lengths and weights into thecontroller. Additionally, a higher performing handheld form-factorsolution is achieved and the size and cost of the active cancellationsystem is further reduced. To describe the features of the presentinvention in more detail, refer now to the following description inconjunction with the accompanying Figures.

System Overview:

FIG. 2 illustrates a system 200 that detects and compensates forunintentional muscle movements in accordance with an embodiment. Thesystem 200 includes a housing 202. The housing 202 includes a subsystem204. The system 200 also includes an attachment arm 206 coupled to thehousing 202. At least one inertial sensor 208 is placed along theattachment arm 206. The attachment arm 206 is configured to accept anobject 210 thereto. The subsystem 204 further includes a portable powersource 212, a motion-generating mechanism 214, a controller 216, acontrol system 218, and at least one distributed motion sensor 220.

The attachment arm 206 can receive the object 210 in a variety of waysincluding but not limited to a friction, snap, or other form of lockingmechanism. The portable power source 212 may utilize a variety ofoptions including but not limited to a rechargeable battery and a solarpanel. The operation and details of the elements of the at least oneinertial sensor 208, at least one distributed motion sensor 220,motion-generating mechanism 214, controller 216, and control system 218will be described in more detail hereinafter.

The at least one inertial sensor 208 and the at least one distributedmotion sensor 220 detect unintentional muscle movements and measuresignals related to these unintentional muscle movements that are createdwhen a user adversely affects motion of the object 210. These sensorsalso detect the motion of the stabilized output relative to the housing202. The control system 218 sends voltage commands in response to thesignals to the motion-generating mechanism 214 through the controller216 to cancel the user's tremors or unintentional muscle movements. Thiscancellation maintains and stabilizes a position of the object 210,keeping it centered relative to the housing 202.

One of ordinary skill in the art readily recognizes that a system andmethod in accordance with the present invention may utilize variousimplementations of the controller 216, at least one inertial sensor 208,at least one distributed motion sensor 220, and control system 218 andthat would be within the spirit and scope of the present invention. Inone embodiment, the controller 216 comprises an electrical systemcapable of producing an electrical response from sensor inputs such as aprogrammable microcontroller or a field-programmable gate array (FPGA).In one embodiment, the controller 216 comprises an 8-bit ATMEGA8Aprogrammable microcontroller manufactured by Atmel due to its overalllow-cost, low-power consumption and ability to be utilized inhigh-volume applications.

In one embodiment, the at least one inertial sensor 208 is a sensorincluding but not limited to an accelerometer, gyroscope, or combinationof the two. In one embodiment, the at least one distributed motionsensor 220 is a contactless position sensor including but not limited toa hall-effect magnetic sensor. In one embodiment, the control system 218is a closed-loop control system.

The closed-loop control system senses motion and acceleration at variouspoints in the system 200 and feeds detailed information into a controlalgorithm that moves the motion-generating mechanism 214 appropriatelyto cancel the net effect of a user's unintentional muscle movements andthus stabilize the position of the object 210. The operation and detailsof the elements of the control system and control algorithm will bedescribed in more detail hereinafter.

Also, one of ordinary skill in the art readily recognizes that a systemand method in accordance with the present invention may utilize avariety of objects including but not limited to kitchen utensils such asspoons and forks, grooming utensils such as make-up applicators, andvarious tools such as manufacturing, surgical and military tools. Thus,the system and method will be useful in not only improving the qualityof life for the multitudes of individuals suffering from neurologicalmotion disorders, but also in assisting in a variety of applicationswhere physiological tremor is an issue including but not limited tomanufacturing, surgical and military applications.

The system 200 stabilizes the object 210′s position about a neutralposition (selected to be θ=0) using the at least one inertial sensor208. To achieve this, the position of the object 210 must be sensedalong with the angle θ. For this position sensing, the at least oneinertial sensor 208 is placed along the attachment arm 206 and is usedto measure the absolute motion of the object 210 while providing lownoise and sufficient sensitivity for the application. The direct sensorplacement of the at least one inertial sensor 208 along the attachmentarm 206 gives a unique advantage to the system 200 as it is extremelyrobust and does not rely on inverse kinematics/dynamics which may changedepending on usage. Thus, as aforementioned, a variety of objects can beused as the object 210 without the need to pre-determine and pre-programthe length and weight of the object 210 into the controller 216.

The at least one distributed motion sensor 220 is located within thehousing 202 which is located at the base of the system 200. The at leastone distributed motion sensor 220 measures the relative motion of theattachment arm 206 relative to the housing 202, wherein the object 210is kept at a center position relative to the housing 202. In oneembodiment, the at least one distributed motion sensor 220 is at leastone custom contactless hall-effect position sensor that provides angularfeedback for the control system 218 and relies on a changing magneticfield that is dependent on the actuation angle.

The changing magnetic field is detected by a strategically placedintegrated circuit (IC) located within the at least one distributedmotion sensor 220, whose analog output is read by the controller 216,providing a completely non-contact angular detection that is capable ofwithstanding a large number of cycles. The at least one distributedmotion sensor 220, with its contactless sensing methods, providessignificantly enhanced reliability over traditional direct-contactsensing methods such as potentiometers that wear over time.

Proper actuator operation is also a key to the overall operation of thesystem 200. Actuator options include SMA wires, piezoelectrics, linearvoice-coils and coreless motors. However, SMA wires, piezoelectrics andlinear voice-coils suffer from various fundamental problems. Forexample, as noted in the “Fatigue Life characterization of shape memoryalloys undergoing thermomechanical cyclic loading” article within the“Smart Structures and Materials” publication, SMA wires suffer fromreliability issues where failures occur after 10⁴ to 10⁵ cycles withstrain amplitudes between 8.3% and 4.4%, which would amount to only 200days usage time. Piezoelectrics, while capable of longer cycle times,are fragile and expensive. In addition, they require high operatingvoltages and thus require relatively large and expensive driveelectronics. Linear voice-coils operate at lower voltages but sufferfrom low force outputs and high costs.

The present invention addresses these drawbacks by using a combinationof coreless micro-motors and miniature gear-reduction systems coupled tothe coreless micro-motors using a coupling mechanism for themotion-generating mechanism 214. In volume, coreless micro-motors areinexpensive and provide up to 1000 hours of operation time. Significantforce of up to 10 newtons (N) can also be produced with these corelessmicro-motors at the required tremor frequency of 0-5 hertz (Hz) throughthe use of a low-cost miniature gear-reduction system, with a totalweight of only 6.5 grams (g). Furthermore, the power drawn from thistechnology is extremely low, estimated at 0.5 watts (W).

The coreless micro-motors are not only capable of holding a maximum loadof 50 g while requiring 0.3 W of power, but are also capable of holdingthe lighter average filled tablespoon load of 14 g while requiring asignificantly lower 0.06 W of power. Thus, the coreless micro-motors aresuitable in generating the required forces for the system 200.

FIG. 3 illustrates a motion-generating mechanism 300 in accordance withan embodiment. The motion-generating mechanism 300 includes a firstminiature gear-reduction system coupled to a first coreless micro-motor302 and a second miniature gear-reduction system coupled to a secondcoreless micro-motor 304. At least one inertial sensor 308 is placedalong an attachment arm 306. The attachment arm 306 is configured toaccept an object 310 thereto.

The first coreless micro-motor is capable of producing rotary motion inthe horizontal (x) direction. This rotary motion is imparted to thesecond coreless micro-motor through a rigid connection that is supportedby a horizontal bearing. The second coreless micro-motor is capable ofproducing motion in the vertical (y) direction. This motion from thesecond coreless micro-motor is supported by a vertical bearing.

A coupling mechanism is used to combine the horizontal and verticalmotions of the two separate coreless micro-motor/miniaturegear-reduction systems 302 and 304. This combination results in abi-directional circular motion of the object 310 (in this embodiment, aspoon). One of ordinary skill in the art readily recognizes that asystem and method in accordance with the present invention may utilize avariety of coupling mechanisms including but not limited to slidingbearing mechanisms, gimbal structures, or bellows structures and thatwould be within the spirit and scope of the present invention.

In the motion-generating mechanism 300, two degrees of freedom aregenerated from the two separate coreless micro-motor/miniaturegear-reduction systems 302 and 304. Additional degrees of freedom (e.g.,a third in the z-direction) can be added to the motion-generatingmechanism 300 by adding motion to the output of the first corelessmicro-motor or the output of the second coreless micro-motor.

System Modeling:

To assist with the development of the control system type and parametervalues, an analytical model of the system 200's properties was created.FIG. 4 illustrates an analytical model 400 in accordance with anembodiment. The analytical model 400 includes a handle 402, an actuator404, an angular sensor 406, an attachment arm 408, an object 410, and aninertial sensor 412. The analytical model 400 was created withsufficient complexity to capture the dynamics of the system 200 and itsresponse when synthesized with a closed-loop control system.

While the system 200 is designed to provide stabilization in multipledirections (e.g., vertical, horizontal, and the z-direction), analysisand modeling in only one direction is required because the motionoutputs were symmetric and completely decoupled from one another. Thus,results from the vertical direction are directly applicable to otherdirections such as but not limited to the horizontal direction, assuminggravitational effects are negligible.

In the analytical model 400, the object 410 moves in the vertical ydirection. The tremor disturbance or unintentional muscle movement(coordinate x) is assumed to act directly on the handle 402. The object410 requiring stabilization (distance l from the base) moves a verticaldistance y. This distance is related to the base coordinate x throughthe transformation,y=x+lθ,   (1)where small angles are assumed. The actuator 404 is capable of movingthe object 410 through the angle θ based on the controller's voltageoutput. The output torque of the actuator 404's coreless motor T isproportional to its armature current i through the relationshipT=K_(t)i,   (2)where K_(t) is a constant. Similarly, the back electromotive force(emf), θ is related to the coreless motor's rotational velocity throughe=K_(ϵ){dot over (θ)}.   (3)

For simplicity, and based on the manufacturer's specifications, K_(e)and K_(t) are approximately equal and are therefore set to a constant k.With the actuator 404's model Equations 2 and 3, the system equationscan be constructed through a combination of Newton's and Kirchhoff'slaws. Through a moment balance the dynamic equation is constructed asI{umlaut over (θ)}+ml/2{umlaut over (x)}=ki.   (4)The second system equation is constructed as

$\begin{matrix}{{{J\frac{\mathbb{d}i}{\mathbb{d}t}} + {Ri}} = {V - {k\overset{.}{\theta}}}} & (5)\end{matrix}$where V is the input voltage/command signal from the controller, J isthe inductance of the actuator 404, and R is the internal resistance ofthe actuator 404.

The system 200 acts as a low-pass filter because it is designed tocancel high-frequency tremor disturbances/unintentional muscle movementswhile retaining low-frequency intended motions. Thus, the system 200 canbe modeled as a transfer function, where an input amplitude X (tremordisturbance) is entered into the system 200, and an output Y (motion ofthe stabilized object) is observed and controlled.

For further analysis on tremor cancellation and to assist in controllerdesign, the system Equations 4 and 5 were transformed into the frequencydomain and manipulated to produce the desired transfer function. Usingthe coordinate transformation Equation 1 and performing a Laplacetransform, Equations 4 and 5 were modified to produce

$\begin{matrix}{{{{\frac{I}{l}{s^{2}\left( {{Y(s)} - {X(s)}} \right)}} + {\frac{ml}{2}s^{2}{X(s)}}} = {{kI}(s)}}{and}} & (6) \\{{{{JsI}(s)} + {{RI}(s)}} = {V - {\frac{Ks}{l}{\left( {{Y(s)} - {X(s)}} \right).}}}} & (7)\end{matrix}$

Solving Equation 7 for l(s) and substituting the result into Equation 6produces a single equation

$\begin{matrix}{{{\frac{I}{l}{s^{2}\left( {{Y(s)} - {X(s)}} \right)}} + {\frac{ml}{2}s^{2}{X(s)}}} = {{k\left( \frac{V - {\frac{Ks}{l}\left( {{Y(s)} - {X(s)}} \right)}}{{Js} + R} \right)}.}} & (8)\end{matrix}$The remaining input in Equation 8 is V, which is the inputvoltage/command signal from the controller. This signal was designed tobe simple in nature to minimize computational requirements and thussignificantly reduce the cost and power consumption of the necessarymicrocontroller.

FIG. 5 illustrates a system diagram 500 of the control system 218 inaccordance with an embodiment. The system diagram 500 includes anunintentional muscle movement 502, a stabilized object 504, accelerationsignals 506, an adaptive acceleration set-point 508, a positionset-point 510, a control algorithm 512, a voltage command output 514, amotion-generating mechanism 516, and position signals 518.

An unintentional muscle movement 502 by a user that adversely affectsthe motion of the stabilized object 504 is detected. Position signals518 relative to the housing are measured by the at least one contactlessposition angular sensor and then are compared to the position set-point510 that is stored in the microcontroller's memory (e.g., ElectricallyErasable Programmable Read-Only Memory (EEPROM)). The position set-point510 is the neutral position of the stabilized object 504 and isinitially calibrated when the system 200 is first activated. Thiscomparison results in a first input signal.

Acceleration signals 506 are measured by the at least one inertialsensor and then are compared to an adaptive acceleration set-point 508.The adaptive acceleration set-point 508 removes the effects of slowchanges in the gravity field due to the changing orientation of thedevice. The adaptive acceleration set-point 508 can be implementedthrough the use of a median filter, low-pass filter, or otherdigital/analog filter capable of removing low frequencies from a signal.This comparison results in a second input signal.

The control algorithm 512 processes the first and second input signalsand sends an appropriate voltage command output 514 to themotion-generating mechanism 516 in each controlled direction to activelycancel the user's unintentional muscle movement and maintain thestabilized object 504.

Based on these two input signals (acceleration signal and angle θ), acontrol law must be constructed for the control algorithm 512. One ofordinary skill in the art readily recognizes that a system and method inaccordance with the present invention may utilize a variety of differentcontrol laws that provide tremor disturbance cancellation while ensuringstability of the object and that would be within the spirit and scope ofthe present invention.

For example, a control law can be derived by applying proportional andderivative gains to the angle θ along with the acceleration signalresulting inV=K ₁ θ−K ₂ ÿ+K3{umlaut over (θ)}.   (9)

In this example, the feedback on the acceleration term provides thedesired low-pass filtering properties. In the exemplified control law(Equation 9), the proportional feedback on the angle θ is applied toallow the device to mimic the function of conventional implements. Thisis achieved by creating “stiffness” in the angular direction to allowthe device to support various loads and while remaining in the neutralposition during the inactive state. Derivative control on the angularinput was selected for stability, particularly to dampen any resonancesintroduced by the proportional feedback on θ. The exemplified controllaw is both effective and computationally simple.

This allows the control algorithm 512 to be implemented in the highlycompact, low-power, and low-cost microcontrollers of the system 200.Substituting the exemplified control law (Equation 9) into V in Equation8 and expanding the terms allows Equation 8 to be expressed as thefollowing transfer function

$\begin{matrix}{\frac{Y(s)}{X(s)} = \frac{n}{d}} & (10)\end{matrix}$where the numerator isn=(2ILJ ² −mL ³ J ²)s ⁴+(4ILJR−2mL ³ JR)s ³+(2K ² LJ+2K ₃ KLJ−mL ³ R²+2ILR ²)s ²+(2K ² LR+2K ₁ KLJ+2K ₃ KLR)s+2K ₁ KLR   (11)and the denominator isd=(2ILJ ²)s ⁴+(2K ₂ KL ² J+4ILJR)s ³+(2K ² LJ+2K ₂ KL ² R+2K ₃ KLJ+2ILR²)s ²+(2K ² LR+2K ₁ KLJ+2K ₃ KLR)s+2K ₁ KLR.   (12)

To reject unintentional muscle movements while retaining intendedmotions, the parameters of the exemplified control law (Equation 9) areoptimized through numerical simulation. For example, this optimizationminimizes the average displacement magnitude of the stabilized object504 (Y, Equation 10) over the unintentional muscle movement frequencyrange of 3-7 Hz, while varying the controller gains K₁, K₂, K₃. Further,in this example, the constraints are defined such that low-frequencymotions in the intended motion frequency range of 0-1 Hz are unaffectedand stability is mathematically ensured. The average phase lag is alsoconstrained to be less than 15 degrees from 0-1 Hz, which is assumed tobe unnoticeable to the user.

For the optimization, computational functions are written to interactwith the trust-region reflective optimization algorithm fmincon inMatlab. The algorithm is run to provide a final solution,K=[121,366,154], which is used for the controller 216 in the system 200.The function has a minimum value of 0.15, which means that the system200 is capable of filtering on average 80% of the input tremordisturbances/unintentional muscle movements in the frequency range of3-7 Hz.

As above described, the system and method in accordance with the presentinvention allow for a highly compact active cancellation approach thatseeks to accommodate a user's tremor by allowing it to exist whilecancelling its effects and stabilizing the position of the object. Byimplementing a motion-generating mechanism to provide the necessaryforces and displacements for tremor cancellation and a control systemand sensor topology to control this motion-generating mechanism, thesystem and method in accordance with the present invention achieve amore robust handheld form-factor with a significantly reduced size andcost.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A system comprising: a housing having an exteriorshape adapted to be held by a user's hand and having an interior volume,the interior volume having an open end; a subsystem positioned in theinterior volume, the subsystem including a motion-generating mechanism,a controller coupled to the motion-generating mechanism, a controlsystem coupled to the controller, and at least one relative-motionsensor coupled to the control system; and an attachment arm having afirst end coupled to the motion-generating mechanism and a second endthat extends through the open end of the housing and is configured toreceive a user assistive device, wherein the attachment arm is movablerelative to the housing, wherein the relative-motion sensor measures theposition of the attachment arm relative to the housing, and wherein atleast one absolute-motion sensor is placed along the attachment arm;wherein the control system uses outputs from the absolute-motion sensorand the relative-motion sensor to stabilize the position of theuser-assistive device by directing the motion-generating mechanism tomove the attachment arm relative to the housing to compensate for theuser's tremors.
 2. The system of claim 1, wherein the subsystem furthercomprises a power source coupled to the motion-generating mechanism. 3.The system of claim 2, wherein the user-assistive device is kept at acenter position relative to the housing.
 4. The system of claim 3,wherein the control system receives a first signal from the at least oneabsolute-motion sensor and a second signal from the at least onerelative-motion sensor, generates voltage commands based upon the firstand the second signals and using the control algorithm, and transmitsthe generated voltage commands to the motion-generating mechanism tostabilize the attachment arm.
 5. The system of claim 4, wherein thegenerated voltage commands are transmitted by the control system and tothe motion-generating mechanism via a controller that is coupled to thecontrol system.
 6. The system of claim 3, wherein the user-assistivedevice comprises any of a manufacturing tool, a surgical tool, a kitchenutensil, a grooming utensil, and a tooth appliance.
 7. The system ofclaim 3, wherein the user-assistive device is coupled to the attachmentarm using a friction mechanism or a snap mechanism.
 8. The system ofclaim 2, wherein the at least one absolute-motion sensor is at least oneinertial sensor with an input that is not limited to angular motionabout an axis and the at least one relative-motion sensor is at leastone contactless position sensor.
 9. The system of claim 2, wherein themotion-generating mechanism comprises at least one motor and at leastone gear-reduction system coupled to the at least one motor.
 10. Thesystem of claim 2, wherein the control system is a closed-loop controlsystem.
 11. The system of claim 2, wherein the at least onerelative-motion sensor relies on a changing magnetic field that isdependent on an actuation angle to provide angular feedback for thecontrol system.
 12. A method for stabilizing a position of an object,the method comprising: providing an apparatus comprising: a housinghaving an exterior shape adapted to be held by a user's hand and havingan interior volume, the interior volume having an open end, wherein thehousing includes a subsystem, a subsystem positioned in the interiorvolume, the subsystem including a motion-generating mechanism, acontroller coupled to the motion-generating mechanism, a control systemcoupled to the controller, and at least one relative-motion sensorcoupled to the control system, and an attachment arm having a first endcoupled to the motion-generating mechanism and a second end that extendsthrough the open end of the housing and is configured to receive a userassistive device, wherein the attachment arm is movable relative to thehousing, wherein the relative-motion sensor measures the position of theattachment arm relative to the housing, and wherein at least oneabsolute-motion sensor is placed along the attachment arm; measuring theabsolute motion of the user-assistive device and the motion of theattachment arm relative to the housing; and directing themotion-generating mechanism to stabilize the position of theuser-assistive device by compensating for the user's tremors by movingthe attachment arm relative to the housing based upon the absolutemotion measurement and the relative motion measurement.
 13. The methodof claim 12, further comprising: comparing at least one signalrepresentative of at least one of the absolute motion measurement andthe relative motion measurement to at least one set-point; andstabilizing the position of the user-assistive device based on thecomparison of the at least one signal to the at least one set-point. 14.The method of claim 13, wherein the subsystem comprises a power source.15. The method of claim 13, wherein stabilizing the position of theuser-assistive device comprises sending voltage commands from thecontrol system to the motion-generating mechanism through thecontroller.
 16. The method of claim 15, wherein sending voltage commandscomprises processing the at least one signal by the control system usinga control algorithm.
 17. The method of claim 13, wherein the at leastone absolute-motion sensor is at least one inertial sensor and the atleast one relative-motion sensor is at least one contactless positionsensor.
 18. The method of claim 13, wherein the motion-generatingmechanism comprises at least one motor and at least one gear-reductionsystem coupled to the at least one motor.
 19. The method of claim 13,wherein the at least one relative-motion sensor relies on a changingmagnetic field that is dependent on an actuation angle to provideangular feedback for the control system.